The most familiar type of venetian blind window covering comprises a series of rigid slats or vanes suspended on ladders that extend from a headrail to a bottomrail. The slats are typically wood, plastic or metal, and rest on the ladder rungs. The slats are tilted from an open position to a closed position by moving the rails of each ladder in opposite directions relative to one another. The entire array can be raised and lowered by lift cords which act to lift a bottom rail.
A modified type of venetian blind is disclosed in U.S. Pat. No. 3,384,519 to Froget. There, the ladder cord system is replaced by front and rear cloth panels to which the vanes are heat welded. Preferably, the cloth panels and vanes are all made of thermoplastic yarns to facilitate bonding by heat welding. Partial rotation of an upper drum, to which the panels are connected, shifts the panels relative to each other thereby causing the vanes to tilt for light control.
Several more recent patents are directed to modifications of Froget's basic idea, wherein the vanes are made of flexible fabric, adhesively bonded to the front and back panels. Additionally, the entire array can be retracted from the window opening by winding it onto an upper wind-up roller by means of cords which lift a bottom rail. Representative of these patents are U.S. Pat. Nos. 5,287,908; 5,313,999; 5,320,154; 5,394,922 and 5,456,304, all assigned to Hunter Douglas, Inc. and U.S. Pat. No. 5,339,882 to Ren Judkins.
These fabric venetians are popular and beautiful products that sell at relatively high prices and incorporate delicate and costly fabrics to achieve the best aesthetic appearance. Such fabric is likely to include flaws such as broken or snagged threads. Such flaws usually are visible in the otherwise uniform and fine mesh of the sheer fabric. Defects such as these are unacceptable in the final product, which is cut from large manufactured bolts of the compound facings-and-strips fabric. Finished-to-size shades must be cut from bolts by positioning to avoid any such flaws, leading to very high waste factors in the cutting of compound fabric bolts. Still, it is not possible to require perfect cloth from the weaver or knitter, and so the waste has contributed greatly to the high cost of these shades.
A second way to manufacture fabric venetians is to use strips in the front panel, the back panel or both panels. U.S. Pat. No. 5,664,613 to Jelic describes another form of construction of a fabric venetian blind. A continuous panel of relatively translucent material forms one facing of the blind. To that panel is bonded a series of parallel fabric strips at uniform intervals. Each strip comprises two adjacent portions of dissimilar material, one being relatively translucent and the other relatively opaque. The free end of the opaque portion is bonded to the facing panel, while the free edge of the translucent portion is bonded to the adjacent strip at the junction line of that strip's two portions. The assembled translucent portions of the constituent strips become the second facing panel of the completed blind. That is, the second facing panel, rather than being a single continuous panel of fabric, is a composite of several parallel strips, each having a height dimension approximately equal to the width (or depth) of the vanes.
U.S. Pat. Nos. 5,490,553 and 5,603,369, assigned to Hunter Douglas, disclose in FIGS. 16-19 a vertical blind wherein each vertical blind is formed of the same strip of material as an adjoining portion of a facing panel on one side of the vanes. The single facing is a composite of narrow vertical strips. The vane portion of this strip is doubled back on itself. Another patent assigned to Hunter Douglas, U.S. Pat. No. 5,638,880, discloses in FIG. 26 a vertical blind with rigid vanes wherein the facing on one side of the vanes comprises a series of vertical strips of material, each joined along one vertical edge to an adjacent strip, and at the other vertical edge to both the adjacent facing strip on that side of a discrete piece of substantially rigid, planar material forming the vane. The patent further discloses that both a front and a rear facing may be fabricated of such strips, and the strips and rigid vanes can be oriented in either the vertical or horizontal directions.
U.S. Pat. No. 4,631,217, also assigned to Hunter Douglas, discloses in FIG. 2 a honeycomb or cellular type of window covering wherein a series of pleated, generally Z-shaped strips, each formed of a single piece of fabric, are bonded together in a manner whereby each strip forms the front portion of one cell, the rear portion of an adjacent cell, and a dividing wall between the two cells. The dividing walls, however, are incapable of functioning as the vanes of a venetian blind type of product because they remain substantially perpendicular to the front and rear faces of the window covering throughout the full range of expansion and collapse of the cells.
More recently, there has been developed a method of formation of fabric Venetian blinds using a technique of helically winding an elongated strip of fabric. This method is the subject of pending U.S. patent application Ser. No. 08/947,608, filed Oct. 9, 1997, and is also disclosed here. In a preferred embodiment a three-portion strip is initially formed from three side-by-side, longitudinal bands or portions of approximately equal width and length. Each side of the central portion is joined by ultrasonic welding to an edge of the adjacent outer portion. The outer portions are formed of sheer or relatively transparent material, and the central portion is relatively opaque. As a result of the helical winding process, the central portion becomes the blind vane, while the two outer portions ultimately become segments of the front and rear composite facings of the blind. Successive windings are staggered and partially overlapped so that two of the three portions of the underlying winding are covered by the overlying winding. Two longitudinally extended glue lines are applied along the underside of the infeeding overlapping strip, these being positioned to bond together adjacent windings. A tube-like wound structure is formed which, when cut open along a line perpendicular to the helix angle, will result in a fabric venetian blind.
The strips of fabric used to form fabric venetian blinds are likely to also include flaws. As in fabric venetians formed from panels, the flaws in fabric venetians formed from strips tend to be quite noticeable. Consequently, there is a need for a method of detecting and removing flaws from strips of material in a manner which minimizes material loss and can be done rapidly using automated equipment. There is also a need to improve the appearance and minimize the waste in manufacturing such shades without losing any of the durability or manufacturing benefits.